Update src/librustc_mir/interpret/intern.rs

Co-Authored-By: Ralf Jung <post@ralfj.de>

Author: oli-obk

src/librustc_mir/interpret/intern.rs

@@ -361,7 +361,8 @@ pub fn intern_const_alloc_recursive<M: CompileTimeMachine<'mir, 'tcx>>(
                 // everything as immutable.
                 // It is UB to mutate through a raw pointer obtained via an immutable reference.
                 // Since all references and pointers inside a promoted must by their very definition
-                // be created from an immutable reference, mutating though them would be UB.
+                // be created from an immutable reference (and promotion also excludes interior
+                // mutability), mutating though them would be UB.
                 // There's no way we can check whether the user is using raw pointers correctly,
                 // so all we can do is mark this as immutable here.
                 InternKind::Promoted => {